Conventionally, as a compact and lightweight reduction gear capable of obtaining a large reduction gear ratio, known is an eccentrically oscillating gear device like the type described in Japanese Patent Application Publication No. 2002-317857. With this kind of gear device, the rotational drive force of the motor is transmitted to the crank shaft (in other words, the eccentric body shaft) via a cylindrical input gear (in other words, an intermediate gear) and a spur gear (in other words, a transmission gear) which engages therewith, and the crank shaft is thereby rotated. The input gear is rotatably supported by the carrier of the reduction gear and the motor flange, respectively, via a bearing.
However, with the gear device configured as described above, since a bearing for supporting the input gear is required, the production cost will increase, and space for placing the bearing inside the carrier is also required.
Thus, as with the gear device described in Japanese Patent Application Publication No. 2007-263253, also proposed is a configuration in which the input gear (in other words, the distribution gear) is not supported by a bearing. In other words, this gear device is configured to define the position of the input gear in the radial direction as well as define the position of the input gear in the axial direction. Specifically, the position of the input gear in the radial direction is defined by a plurality of (for instance, three or more) spur gear (in other words, the eccentric body shaft gear) disposed around the input gear engaging with that input gear, and the position of the input gear in the axial direction is defined as a result of the input gear being disposed between the two external gears inside the carrier. With this kind of configuration, a bearing for supporting the input gear is no longer required.
However, with the gear device described in Japanese Patent Application Publication No. 2007-263253, since the structure for regulating the movement of the input gear in the axial direction requires the input gear to be disposed inside the carrier of the gear device and the input gear to be disposed between two external gears, the structure becomes complicated.
Furthermore, with this kind of gear device, upon replacing the input gear in order to change the reduction gear ratio or the like, work for disassembling the carrier, in which the mechanical portions of the gear device are concentrated, and replacing the input gear inside the carrier is required. Thus, a complicated work process will arise, and there is a possibility that the production cost, maintenance cost and the like may increase.